Thermochemistry 4
Boon Chemistry February 4 & 5, 2013
Catalyst
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Take out your homework
please.
According to the heat curve for
 Objectives
• I can calculate the amount of
water shown below:
energy transferred as heat during a
Does the system absorb or release
phase change.
heat as ice melts?
Does the temperature of water
 Agenda
increase when it is melting?
 Catalyst
What phase change happens at #4?
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Exit Slip Review
Notes: Latent Heat of Phase
Change
Practice Calculations and
heat curve analysis
Exit Slip: Calculations
Phase Diagram Review
#5: When
#2:
The temperature
water freezes
remains
the kinetic
constant
energy
whenofthe
water is a mixture
molecules
decreases.
of ice
Theand
molecules
water because
move the
energy isThis
slower.
being
is exothermic
used to meltbecause
the ice.energy is
released.
(1) Ice; temperature is
increasing
(2) Ice and water;
temperature is constant
(3) water; temperature is
increasing
(4) Water & steam;
temperature is constant
#4: When water boils the kinetic energy of
molecules increases. The molecules move faster.
This is endothermic because it requires energy.
(5) steam; temperature is
increasing
Exit Slip Review
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7. Which of the following statements best describes the relationship
between the transfer of heat and temperature?
(a) The transfer of heat always raises temperature.
(b) The transfer of heat never causes a change in temperature.
(c) The transfer of heat always decreases temperature.
(d) The transfer of heat does not always cause a change in
temperature.
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8. What happens to the temperature of matter as energy is added
during a change of state?
(a) temperature increases
(b) temperature decreases
(c) temperature does not change
(d) temperature changes depending on the material
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9. List the names two phase changes that result in a
release of energy to the surroundings:
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Freezing, condensation, deposition
10. List the names of two phase changes that result in an
absorption of energy from the surroundings:
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Vaporization, fusion, sublimation
Exit Slip Review
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Exothermic
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2NaHCO3 + energy→ Na2Co3 + H2O + CO2
Endothermic
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Exit Slip Review
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3. In an endothermic process, energy is (absorbed/released).
4. In an exothermic process, energy is (absorbed/released).
5. Which of the following is an example of energy:
(a) color
(c) copper
(b) heat
(d) temperature
Why? Heat is one way that energy is transferred. Temperature is a
measure of the average kinetic energy of particles in an object. But it is not
energy itself.
6. When energy is added to liquid water, what happens?
(a) The molecules move and vibrate less.
(b) There is no change in the motion of the molecules.
(c) The molecules move and vibrate more.
(d) The water becomes ice.
Specific Heat Review:
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Use your notes to do the following specific heat problem.
3000 g of water are heated from 50 °C to 99°C. The
specific heat of water is 4.18 J/g*K. How much heat was
absorbed?
Notes: Heat Transfer During Phase
Changes
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We discussed the fact that temperature does not increase
during a phase change. Instead, all the energy that is
released or absorbed is used to power the phase change.
On the phase change diagram this portion of the heating
curve has a slope of zero.
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Today we will calculate how much heat is absorbed or
released during a phase change. We will use water as our
example, but a similar calculation can be done for other
substances.
What do you think? Which requires more
energy to melt?
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The amount of heat absorbed or released during a phase
change is generally called the latent heat of
transformation. Different substances have different latent
heats of transformation. Also, different phase changes
absorb or release different amounts of heat. For example,
less energy is absorbed to melt ice than is absorbed when
water is vaporized. Table 1 contains the heats of
transformation for water.
Analyzing Data.
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How do the heats of
transformation for water
compare to the specific heat of
water?
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How many joules are released
when 1 gram of water freezes?
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Which requires more energy:
melting or vaporization?
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Why is the heat of fusion the
same as the heat of
crystallization?
Latent Heat of Transformation
Calculations
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If we know the latent heat of transformation of a
substance, we can calculate the amount of heat energy
absorbed or released during a phase change.
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The formula:
Q = m × ΔHtranformation
Phase Change Calculations
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Example 1: How much heat is required to melt 1000 g of
ice at 0°C?
Phase Change Calculations
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Example 2: How much heat is needed to vaporize 200 g
of water at 100°C?
Heat Transfer Calculations
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Example 3: How much ice, at 0°C, can be melted by 100 J
of heat?
Work Time
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1. Complete today’s notes worksheet
2. Get a stamp when you are done and check your
answers.
3. Work on the phase change diagram worksheet (passed
out today). This is due Friday.
4. Work on any worksheets that you have not completed
from last week. These are due Friday.
Exit Slip
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Expectations:
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Tools:
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You may use all your notes, worksheets, and flash cards.
You may use your own calculator.
What do I turn in?
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You will work silently and independently.
When you are done, cover your exit slip with your handouts.
You will turn in your exit slip only.
Homework:
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Complete any worksheets or article questions that you have
not finished.
Homework
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Due Thurs/Fri (Feb. 7 & 8): All Thermochemistry notes
and handouts.